Linear accelerator



LINEAR ACCELERATOR N. C. CHRISTOFILOS ET AL Feb; 17, 1959 Filed June 5, 1957 INVENTOR. v NICHOLAS c. CHRISTOFILOS IRVING J. POLK BY om I l lull I I'll g @m @N omm mm 6 ow Nw N. c. CHRISTOFILOS ET AL 2,874,326

Feb. 17, 1959 LINEAR ACCELERATOR 3 Sheets-Sheet 2 Filed June 5, 1957 INVENTOR. NICHOLAS C. CHRISTOFILOS IRVING J. POLK Feb. 17, 1959 N. c. CHRISTOFILOS E TAL- 2, 7

I LINEAR ACCELERATOR Filed June 5, 1957 v 3 Sheets-Sheet 3 United States Patent LINEAR ACCELERATORv Nicholas C. Christofilos, Berkeley, Calif and Irving J. Polk, Patchogue, N. Y., assignors to the United States of America as represented by the United States Atomic Energy Commission Application June 5, 1957, Serial No. 663,859- 13 Claims. (Cl. 315-551) This invention relates to improvements in linear particle accelerators and more particularly to a new type of drift-tube system for a linear ion accelerator.

A drift-tube system for a linear ion accelerator comprises an array of drift tubes and a resonant cavity in which it is disposed. Heretofore, a conventional linear ion accelerator as disclosed in Patent No. 2,545,595, issued March 20, 1951 to L. W. Alvarez, has incorporated therein a drift-tube system but this type has certain disadvantages. Some of these disadvantages are discussed below.

A disadvantage of the conventional linear ion accelerator has been that optimum shunt impedance therefor has not been achieved. Power consumption is conveniently described in terms of its shunt impedance which should be as high as possible. It is given by the ratio of the square of the ion energy gain per unit length in volts to the radiofrequency power dissipation per unit length in Watts. An optimum value for the shunt impedance has not been achieved for linear ion accelerators as there have been losses due to the shape of the drift tubes. The shape used until now has conformed to a right circular cylinder.

Another disadvantage of the conventional linear ion accelerator has been the considerable shut-down time due to arcing breakdown between adjacent drift tubes. This is attributable to the high field intensities on the outer surfaces of the drift tubes.

Still another disadvantage of the conventional linear ion accelerator has been the use of grids or foils for focusing the ion beam. It is Well known that without grids or foils disposed in the ion beam, no other focusing system for the ions being present, phase stability of the beam and radial focusing cannot be achieved simultaneously. If either phase stability or radial focusing is lacking, the ion output of the accelerator is limited in its energy and in its current. The use of grids or foils in the upstream ends of the drift tubes (i. e. toward the ion source) to achieve radial focusing undesirably attenuates the beam of particles and also introduces mechanical difiiculties in the mounting and the handling of extremely thin sheets of metal. It is often difficult and sometimes impossible to incorporate in the drift tubes a focusing system to compensate for. the radial defocusing which occurs in the gaps because of the size and complexity of the focusing apparatus. This. problem is intensified by the requirement of making the diameter of each drift tube progressively smaller along the ion path in order to establish the desired electric field pattern in the resonant cavity.

A further disadvantage of the conventional linear ion accelerator has been the necessity of changing the resonant cavity diameter in steps alongthe 'ion path toincrease the drift tube diameter. This is often necessary when the length of the acceleratoris increased in order P s a le 5 e si st. h hsre rs ne.

put therein and is usually done by the use of separate cavities within a separate containing tank therefor.

The present invention overcomes the aforementioned 'difiiculties by providing a new type of drift-tube system for a linear ion accelerator. To use this invention to its greatest advantage, modifications are also made in the. design of conventional accelerators. The first modification reduces the gap capacity between adjacent drift tube ends. This is accomplished by reducing the ratio of the diameter of the drift tube to the diameter of the resonant cavity. The desired ratio may be achieved either by. decreasing the drift-tube diameter, increasing the cavity diameter or doing both. The second modification which. is carried out to complement the first reduces the concentration of the magnetic field intensity at the longitudinal. midpoin of the external surface of each drift tube. This is accomplished by increasing the external drift-tube diameter at the longitudinal center region. This procedure permits a resonant cavity of extensive length with constant diameter.

The new typeof drift-tube system for a linear accelerator in accordance with this invention. has the optimum. shunt impedance for a given cavity diameter, gap spacing between drift tubes, length of unit cell and free-space, wave length of the radiofrequency field with which the cavity is energized. The outer surface of each drift tube conforms to a surface of revolution symmetrical with regard to a plane perpendicular to the axis of revolution at the midpoint of the drift tube; the drift tube is quasiellipsoidal in appearance. The trace of the surface in an axial plane is a curvewhich may be determined by numerical integration of the equation:

tie E, where r and z are cylindrical coordinates of the curvev with the origin thereof taken on the axis of the drift tube at the gap mid-point and E and E are the radial and, axial components of the electric field. Thefieldsmay be expressed as a finite number of cylindrical harmonics. T henumerical integration aforesaid-may conveniently be carried out on a high-speed digital computer. It is not intended that mention of the possibility of calculating.

" the surface contourof each drife tube limits in any Wayance of a quasi-ellipse.

the application of drift tubes in accordance with the. present invention. 1

Therefore, the present invention involves an apparatus. for accelerating high-velocity ions. The, apparatus includes a new drift-tubesystem and a cylindrical resonant cavity of uniform diameter-energized by a radiofrequency generator for producing a longitudinal electric field there-I in. Mounted in the cavity is a series of drift tubes having curved outer surfaces, in accordance withthe inventioni The drift tubes are disposedvwithin the cavity alongthelongitudinal axis thereof with gaps between them. The

tubes increase in length along the direction of movement the drift tube surface in anaxialrplane has the appearlength. of the tubesand the radiusof curvature of th e mid outer portion of the curve increases with increasing I length of the tubes. The radius of curvature of substantially all of the curve between the end and mid portions changes continuously and gradually therebetween.

Therefore, it is an object of the present invention to provide a new and improved linear accelerator for charged particles.

Anotherv object of the present invention is to provide a newdrift tube for alinear accelerator.

Further, the radius of curvature of the end portions of the curve decreases with-increasing Other objects and advantages will be apparent in the following description and claims considered together with the accompanying drawings in which:

Figure l is a schematic vertical elevation, partly in cross section, of a linear accelerator incorporating the improved drift-tube system of this invention.

Figure 2 is an enlarged cross section of Figure 1 taken on the'line 2--2 showing schematically the manner of mounting the drift tubes and supporting the tank on linear bearings.

Figure 3 is a partial, vertical, mainly axial cross section of a typical drift tube taken on the line 3-3 of Figure 2'showing the contour of the drift tube and an alternating-gradient electromagnetic focusing assembly.

Figure 4 is a vertical cross section taken on the line 4-4 of Figure 3, of a typical drift tube and shows the contour of the drift tube as viewed from the axis of the resonant cavity and shows also the nature of the laminations of the electromagnet quadrupole.

Figure 5 is an elevation, mainly in section, showing a typical vertical adjustment assembly; this is taken on the line 5-5 of Figure 1.

Figure 6 is a plan view, turned 90 clockwise and mainly in section, of a typical horizontal adjustment assembly taken on an axialplane through the line 6-6 of Figure 2.

Figure 7 is a diagrammatic representation of the outline of a drift tube in accordance with the invention and is used to illustrate a method of deriving the surface contour.

Referring to the drawings and particularly to Figures 1 and 2, a linear ion accelerator incorporating the drifttube system of the present invention comprises a plurality of tank sections 10 joined together at flanges 12 by bolts 14. Shims (not shown) are held in place between flanges '12 by the bolts 14 to permit accurate axial alignment of the tank sections 10 to form a continuous tank 16. The inner volume of tank 16 is the resonant cavity 18. Tank sections 10' are made, for example, of lowcarbon steel, copper clad on their inner surfaces to provide good radiofrequency conductivity. For a representative arrangement of the accelerator, the tank 16 may be 110' long and have eleven similar tank sections, each 10 long. The cavity so formed is designed to operate at approximately 200 megacycles to produce a beam of protons having energies of the order of 50 million electron volts.

Tank 16 is supported on steel piles (not shown) by a linear bearing assembly 20. Linear bearing assembly 20 comprises two linear bearings 22 each mounted on a linear bearing support rod 24. Each support rod 24 is mounted by uprights 26 on a base plate 28 which is supported upon a concrete pile cap 30. The linear bearings 22 support transverse tank base plates 32 on which the tank sections 10 are supported by tank legs 34.

The resonant cavitymust be vacuum-tight and a vacuum is produced in resonant cavity 18 by vacuum pump assemblies 36 joined to ports 38 in tank 16 by flanges 40.

An array of the new drift tubes 42 is mounted axially within resonant cavity 18 and each tube is supported by a vertical stem 44 and horizontal stem 46. Vertical stem 44 is mounted on tank 16 by vertical stem support assembly 48 (Figure 5) and horizontal stem 46 is mounted on tank 16 by a horizontal stern support assembly 50 (Figure 6). Drift tubes 42 have stainless steel cylindrical passageways 52 therein, as shown more clearly in Figures Land-,4. The, length of passageway 52 within drift tubes 42 increases in length from drift tube to" drift tube along the ion path in resonant cavity 18 in accordance with the requirement for phase stability of ions. To eliminate the possibility of arcing at the first gap, the entrance drift tube 6 is made as one-half drift tube and is mounted on the entrance end plate 8.

A radiofrequency generator 54 (Figure 1) sets up the accelerating electric field for ions in the resonant cavity 18. To assist in tuning the resonant cavity 18 to resonance at the design frequency, a plurality of spherical copper ball tuners (not shown) are mounted therein via radial copper stems, not shown. Each spherical copper ball tuner has its center in a radial plane through the center of a gap between adjacent drift tubes 42 and is entirely outside the gap.

Ions are injected into tank 16 via passageway 56 by an ion source 58. Ion source 58 comprises ion injector 60 which is a Cockroft-Walton accelerator for the representative arrangement described herein. Ions from injector 60 pass through conventional ion buncher 62 and enter the resonant cavity 18 properly bunched. The buncher 62 comprises an open tube 61 in which the accelerated ions overtake the decelerated ions and a resonant cavity 63 properly phased at the frequency of the resonant cavity 18.

In order to remove heat that is evolved in tank 16 and provide temperature control for the resonant cavity 18 there is provided a thermostatically controlled water cooling system, not shown. The system comprises a plurality of axial hairpin tubes bonded to tank sections 10 and looped at the flanges 12; near the ion source, end of tank 16 each pair of adjacent pipes is formed in a loop for the return. At the ion exit end of resonant cavity 18 alternate members of each pair of pipes are joined to water manifolds. Such a cooling system readily permits keeping the tank 16 at a constant average temperature.

The nature and manner of construction of a typical drift tube will be understood by reference to Figures 4 and 5. The outer surface 70 of drift tube 42 is formed from copper shell 72 and copper front nose plate 74 and copper rear nose plate 76.

The trace of outer surface 78 is an axial plane, as shown in Figure 7, conforms to quasi-ellipse 78. The horizontal axis 79 of the quasi-ellipse 78 lies along the axis of resonant chamber 18 and its vertical axis 81 is perpendicular thereto. The quasi-ellipse is symmetrical about its axes. The curve of quasi-ellipse 78 is divided generally into an end-portion 80, a middle portion 82 and an intermediate portion 84. The radius of curvature of end-portion decreases with increasing length of pas-' sageway 52 along the ion path. The radius of curvature of mid-portion 82 increases in length with increase of passageway 52 along the ion path. The radius of curvature of the intermediate portion 84, with the exception noted hereafter, changes smoothly and continuously between endportion 80 and mid-portion 82. The quasi-ellipse 78 for the typical drift tube 42 shown in Figures 3 and 4 was calculated by numerical integration on a high-speed digital computer of the equation dz E,

where E and 13,, the axial and radial components of the electric field respectively are defined by two sets of equations of a finite number of cylindrical harmonics. The two sets of equations are necessary, as the requirement that E =0 at the tank wall can be accomplished only by employing the Neumann function. This is due to the fact that because of the capacitative load of the drift tubes, the tank radius must be less than the radius given by the first zero of the Bessel (I function. Consequently, to make E zero at the outer radius it is necessary to include the Neumann function which becomes infinite on the axis. Hence different solutions must be used for the axis. These solutions will be matched at an intermediate aezasae where J and Y are the Bessel and Neumann functions respectively K (kr)=i(1r/2)H (ikl) I (kr) =J( ikr) k =Z1r/L k (.uk -k In principle the matching radius, a, is arbitrarily chosen but in practice it is found that its value must lie between rather close limits. At the matching radius it is possible to match exactly two components: the radial component of the electric field and the azimuthal component of the magnetic field. Then the average value of the other component (the axial electric field) across the gap is matched thus:

From the requirement that B and H are to be identical functions of 2 at the matching radius a, We determine the constants .L mks s ,5 nm Three conditions remain to be satisfied 2 r/ d 1 f E. z 1 5).

2 0/ 1 f E dz 1 s) E =0 at r=a, z=g/2 above and E and E. are known, it is possible to proceed with numerical integration of Equation 1. By choosing m=3, one acquires one more degree of freedom and thus the shunt impedance can be optimized.

Referring again to Figures 3 and 4, the cylindrical copper inner liner 8 8 is furnace brazed to outer shell 72. and forms a water-tight annulus 9% in cooperation with it. Vertical stem 44, which may be stainless steel pipe copper-clad on its outside, is leak-tight furnace brazed into an opening in outer shell '72 and serves as a passage 92 for cooling water. Horizontal stem 46, which also may be a stainless steel pipe copper-clad on its outside, is also leak-tight furnace brazed into an opening through outer shell 72. Coaxially within stem 46 there is an electrical conduit 96 also preferably of stainless steel. Coolthe annulus 96 and leaves through the vertical stem passage 92.

The rear nose plate 76 and forward nose plate 74 are furnace brazed to outer shell 72 and to downstream portion 98 and upstream portion 112, respectively, of inner passageway 52. The two portions of the passageway 52 are overlapped and joined as with soft solder.

Electromagnet quadrupole assembly 10% is mounted in inner liner 88 by means of a collar 1132 held in place by bolts 104; threaded into the liner. The quadrupole assembly comprises a plurality of magnet laminations 16 6 and electrical windings 168 constructed as a unit in accordance with conventional electric motor technique. Energizing electric wires 11%) connect with electromagnet via electric conduit 96. After electromagnet quadrupole assembly is secured within shell'72, the front nose,

plate 74, sealed to left portion 112 of inner passageway 52, is secured, as by copper welding, to outer shell 72.

' Figures 5 and 6 show in detail the vertical stem support assembly 48 and horizontal stem support assembly 50, respectively. These supports are arranged so each drift tube can be adjusted in position and then maintained in its final alignment by shims. techniques are'used to align the magnetic axes of the focusing magnets along the axis of the tank.

The vertical stern support assembly 48 includes a stem box 120, which is mounted axially on tank section 10 and spans substantially. the entire distance between its flanges 12. Stem box has a cover plate secured by bolts 184, compressing a dumb-bell-shaped gasket 186' The enclosure,

to form a hermetically sealed enclosure. is separately evacuated by a vacuum sysem, not shown. Vertical stem 44 is positioned vertically by retaining ring 122 bearingupon a washer 124 which is in turn supported by spherical bearing. 126. The bearing, in turn, is supported upon a finely machined annular positioning shim 128 resting upon an annular shoulder formed in the base of stem box 120. The shim 128 has an enlarged central opening.

Spherical bearing 126 has an upper and lower. part. The, upper part has a spherical convexsurface which fits in a matching spherical concavity of the lower part and Conventional optical 7 permits axial and lateral movement of stem 44 in the opening through shim 128 and thus of drift tube 42 during positioning thereof.

Vertical stem 44 comprises a lower vertical stem portion 130 and an upper vertical stem portion 132. A bushing 134 is furnace brazed to the lower extremity of upper vertical stem portion 132. An open ended coupling or socket 136 is furnace brazed to the upper extremity of lower vertical stem portion 130. The bushing 134 is inserted into and threaded to coupling 136 thereby making a firm joint tying together the lower vertical stem portion 130 and the upper vertical stem portion 132. A leak-tight seal is made between bushing 134 and coupling 136 via an O-ring 139.

The interior of the tank is lined with a copper wall 138 to form the wall of the resonant cavity. A bellows assembly 137 provides good radiofrequency electrical contact between lower vertical stern portion 130 and the wall 138. Bellows assembly 137 comprises a bellows 140, preferably of copper, which is furnace brazed at its upper extremity to a collar 142, preferably of copper. Collar 142 is firmly secured in an opening through the wall of the tank by means of a bushing 144 threaded into it. The bushing 144 extends through an opening in the wall of tank and terminates in a flange 145 at its outer end. Bushing 144 rests on headless set screws 152 threaded into its flange 145; the screws 152 press on a steel bearing ring 154 seated in a shallow annular recess in the outer surface of tank 10.

The collar 142 is vacuum sealed against copper wall 138 by an O-ring 146 in a shallow recess in copper liner 138. Good radiofrequency electrical contact is made between collar 142 and copper wall 138 by spring ring 148 between them in the same recess. Radiofrequency electrical contact is made between bellows 140 and vertical stem portion 130 through a ring 150, of copper preferably, which is itself furnace brazed to vertical stem portion 130.

Water for cooling drift tube 42 exits from upper vertical stem portion 132 via exit tube 156. Exit tubing 156 is sealed into a flanged bushing 158 which is held firmly in an opening in stem box 120 by means of a vacuum seal 160 and the bushing flange. The bushing 158 is secured in position by a nut 162 threaded on the inner end of the bushing against the wall of the stem box. A bellows 164 connects bushing 158 and a 90 conduit coupling which, in turn, is sealed to the upper end of upper vertical stern portion 132. The seal is made by means of an O-ring 168 and a demountable clamp assembly 170 which has a screw 176 and a block 178 bearing on a surface of coupling 166 axially with respect to the stem. A lock nut 177 threaded on the screw 176 secures the assembly firmly.

The apparatus associated with horizontal stem 46 is somewhat similar to that employed with the vertical stern. Horizontal stem 46 makes radiofrequency electrical contact with inner wall 138 via a bellows assembly 200 preferably all of copper. Bellows assembly 200 comprises a bellows 202 sealed at its inner extremity to a ring 204 which is, in turn, brazed to horizontal stem 46. At its outer extremity bellows 202 is brazed to a collar 206 which, in turn, is sealed to wall 138 by an O-ring 208 and makes radiofrequency electrical contact therewith by spring ring 210. This construction is substantially the same as that described with respect to bellows 140 and collar 142.

Collar 206 has a cylindrical extension 207 extending outward through an opening in tank 10. To support collar 206 against inner wall 138 of tank 10, the extension 207 is threaded at its outer end into an internally threaded ring 216. In a manner similar to flange 145 on bushing 144, the ring 216 has a series of peripherally spaced, retaining set screws 212 threaded therein and bearing on an annular washer 214 in a shallow recessin the wall of .tank section .10. Threading the ring 216 onto extension 207 and tightening retaining set screws 212 causes collar 206 to be pulled against wall 138. i

The electrical and cooling water connections for the horizontal stem are contained in a horizontal stem box 218 which is generally similar to the vertical stem box 120. Box 218 is similar to a structural channel, having a base welded to the tank wall and side walls; it is generally rectangular in cross section. A removable cover is bolted on against a dumb-bell-shaped gasket as described with respect to box 120.

The horizontal stern box 218 contains an assembly for conveying drift tube cooling water flowing inward through the annular passage 94 between horizontal stem 46 and conduit 96. The assembly includes a stem block 228 which is generally cylindrical and coaxial with stem 46. The block 228 is bored to receive the electrical conduit 96 which is sealed to block 228 at its outer end. The block 228 has a counter-bore 229, to provide a passage for cooling water continuous with the passage 94. At its inner end the block 228 is further counter-bored to receive the outer end of stem 46. A seal is provided at this point as by brazing.

The passage into the counter-bore 229 in block 228 includes a bushing 226 inserted in a radial opening in block 228, the opening terminating at the counter-bore 229. Bushing 226 is sealed into the opening by means of an O-ring gasket 230 and held in place by a clamp 232. The side wall of box 218 directly opposite bushing 226 has an opening to receive a bushing 220 and the bushings 220 and 226 are connected by a bellows 223 which may be of brass. The bushing 220 is threaded at its inner end and there receives a nut 224. At its outer end bushing 220 is flanged and an O-ring gasket 232 is compressed between the flange and the side wallof box 218 when the nut 224 is tightened.

The stem block 228 is pivoted to permit the horizontal stem 46 to rock when adjustment in-the vertical position of the drift tube is made. The pivot assembly includes a pivot block 234 secured to the outer end of block 224 by screws 236, only one of which is shown. The pivot block 234 is provided, at opposite ends, with stub shafts 238 transverse to the stem 46, and the stub shafts are supported in V-shapecl notches or bearings 240 in the arms of a U-bracket 242. The stub shafts 238 are held in the bearings by means of a clamp member 244 bolted to each arm of the U-bracket 242 across the stub shaft. 7

As in the case of the vertical stem, the horizontal stem is also provided with means for adjusting its position and alignment in order to locate the drift tube properly. The U-bracket 242, which carries the stem 46 through the block 228, is secured to the floor of the box 218 by means of cap screws 250, the U-bracket being open at the base about the block 228. Screws 250 are threaded directly into the floor of the box'218 which is recessed. Between the base of bracket 242 and the recessed floor of the box there is located rectangular shims 244 secured in place by screws 250. The shims 244 fix the horizontal position of the drift tube with respect to the axis of the resonant cavity. For axial alignment of the horizontal stern and thus of the drift tube, a shim or shims 252 may be bolted to one or both of the arms of U-bracket 242 between the stem block 228 and the arms.

In the wall opposite the bushing 220, the stem box is provided with a conventional, vacuum-tight electrical coupling 256 sealed to the wall by means of an O-ring 260. The connecting wires between coupling 256 and the drift tube are fastened at one end to the coupling lugs. A slot 254 milled in the top of the block 228 and communicating with the interior of the electrical conduit 96, provides the passage for the wires into the stern.

linear accelerator mentioned above, has performance superior to that of a conventional proton linear accelerator designed to operate at the same energy. The beam ets t s ncre sed approximately 300 times; the. shunt. impedance is approximately 20% greater; the power loss resulting from the shape of the drift tube system is. reduced approximately 40%. Because the invention per mits a singleresonant cavity of constant diameter, rather. than several cavities of different diameters as have been previously required, advantages of simplicity in design, construction and operation of the accelerator are achieved. Some of the details of the design of the representative arrangement are as follows: V

The Cockroft-Walton accelerator aforementioned used as the ion injector provides 0.75 million electron-volt protons. The drift tube system designed to accelerate protons of this initial energy has 124 drift tubes whose vertical. axes form a sequence in which there is a first minimum length of 20.3 cm. for the first drift tube, a second minimum length of 14.7 cm. for the last drift tube and a maximum length of 25.1 cm. for a drift tube therebetween. The curvature of the envelope of the drift tube surfaces varies smoothly and continuously from the first tube to the last tube. i

In order to compensate for any misalignment of the drift tubes, the drift tubes are divided into four. groups with each group having a different diameter for its ion passageways. From the 0.75 m. e. v. injection point to 1.5 m. e. v., the diameter is 1.3 cm.; from 1.5 m. e. V; to 3.0 m. e. v., the diameter is 1.9 cm.; from 3.0 m. e. v. to 6.0 m. e. v., the diameter is 2 .5 cm.; and from 6.0 m. e. v. to 50m. e. v., the diameter 3.2 cm.

The vertical stem is a light pipe 1 cm. in diameter whereas the horizontal stem. is a relatively heavy pipe 3 cm. in diameter. The temperature of the tank is held" constant to about 0.1 C. by the thermostatically controlled water cooling system. The electromagnet quadrupoles have their laminations and windings bonded byresin 101 (Figure 4). They provide field gradients at the low energy end of the accelerator of about 6000 gauss per cm. which diminish to about one tenth ofthat. value at the high energy end.

Because the distance between adjacent horizontal stems is short in the first tank section, it has proved convenient to mount alternate horizontal stems of the first 20 drift tubes from the opposite sides of the tank. They are supported by a horizontal stem support assembly similar to the one aforedescribed.

While the salient features of this invention have been described in detail with respect to one embodiment, it will, of course, be apparent that numerous modifications may be made within thespirit and scope of this invention, and it is therefore not desired to limit the. invention to theexact details shown except insofar as they may be defined in the following claims.

We claim: 7

1. An apparatus for accelerating high velocity ions comprising a cylindrical resonant cavity of constant diameter, a radiofrequency generator for energizing said cavity to produce a longitudinal field therein, ion source means coupled to said cavity to inject ions thereintofor longitudinal acceleration by. said field, a series of drift tubes with cylindrical longitudinal bores and curved outer surfaces spaced apart to form gaps disposed within said cavity along the longitudinal axis thereof, said tubes increasing in length along the direction of movement of ions through said cavity, substantially all of the outer surface of each of said tubes having an ellipsoidal curvature, the radius of curvature of the end portions of said surface decreasing with increasing length of said tubes, the radius of curvature of the middle portions increasing with increasing length of said tubes, and the radius of curvature of substantially all of the surface intermediate the end and middle portions changing continuously and gradually therebetween.

2. The apparatus of claim 1 in whichthe drift tubes have their vertical axes forming a sequence, said sequence having a first'minimurn length at the first drift tube of. said cavity, a second minimum length at the last drift tube of said cavity and a maximum lengthv there between, the envelope of the surfaces of said drift. tubes having a curvature. which varies continuously and smoothly from said first drift tube to said last drift tube.

3. An apparatus for accelerating high velocity ions comprising a cylindrical resonant cavity of constant diam} eter, a radiofrequency generator for energizing said cavity to produce a longitudinal field therein, ion source means for producing a longitudinally directed bunched beani of ions in said cavity, said ions being accelerated in the longitudinal direction by said field a series of drift tubes with cylindrical longitudinal bores and curved outer surfaces spaced apart to form gaps disposed within said cavity along the longitudinal axis thereof, said tubes increasing in length along the direction of movement of ions through said cavity, substantially all of the outer surface of each of said tubes having a ellipsoidal curvature, the? radius of curvature of the. end portions of said surface decreasing with increasing length of said tubes, the radius of curvature of the middle portion increasing with increasing length of said tubes, and the'radius of curva ture of substantially all. of the surface intermediate the .end and middle portions changing continuously and gradually therebetween, and an alternating gradient focus ing system within said drift tubes to focus said beam of ions in passage therethrough.

4. The apparatus of claim 3 wherein the alternating gradient focusing system includes at least one quadrupole magnet within each of said drift tubes for establishing. a strong focusing magnetic field longitudinally therethrough to simultaneously radially focus and phase stabilize the 'beam of ions accelerated therethrough.

The apparatus of claim 3 in which the alternating gradient focusing system includes an electromagnet quadrupole assembly within each of said drift tubes, said electromagnet quadrupole assembly including a plurality. of magnet laminations and an electrical winding to produce an alternatinggra'dient magnetic focusing field within said drift'tubes. whereby said beam of ions. is radially focused and phase stabilized during acceleration therethrough.

6-.An apparatus for accelerating high velocity ions comprising a cylindrical resonant cavity of constant diameter, a radiofrequency generator for energizing said cavity to produce a longitudinal field therein, ion source means commuuicably coupledto one end of said cavity and directing a bunched beam of ions along the axis thereof for longitudinal acceleration by said field, a series. of drift tubes with curved outersurfaces spaced apart to form gaps disposed within said cavity along the longitudinal axis thereof, said tubes increasing in length along the. direction'of movement of ions through said cavity, substantially all of the outer surface of-each of said tubes having. an ellipsoidal curvature, the radius of curvature of the end portions of said surface decreasing with increasing length of said tubes, the radius of curvature of the middle portion increasing with increasing length of said tubes, and the radius of curvature of substantially all of the surface intermediate the end and middle portions changing continuously and gradually therebetween, each of said drift tubes including an outer shell, a cylindrical inner liner within said outer shell forming a cooling water. annulus therebetween, an entrance Water passageway in said shell communicating with said water annulus, an exit water passageway in said shell communicating with said water annulus, means for introducing cooling waterinto said entrance water passageway from outside of said resonant cavity, means for conveying cooling water from said water exit passageway to the outside of said resonant cavity, a forward nose plate and a rear nose plate secured to said outer shell to form therewith. said outer surface of said drift tube, anion passage? way;,in said drift tube, said ion passageway. haviug an upstreamportion and a downstream portion, said; for;

games ward nose plate being attached to said upstream portion and said rear nose plate being attached to said downstream portion.

' 7. An apparatus for accelerating high velocity ions comprising a tank having one end adapted for attachment to a source of longitudinally directed ions, a cylindrical resonant cavity of constant diameter within said tank, the inner surface of said tank being the wall of said cavity, a radiofrequency generator for energizing said cavity to produce a longitudinal field therein, a series of drift tubes with curved outer surfaces spaced apart to form gaps disposed within said cavity along the longitudinal axis thereof, said tubes increasing in length along the direction of movement of ions through said cavity, substantially all of the outer surface of each of said tubes having an ellipsoidal curvature, the radius of curvature of the end portions of said surface decreasing with increasing length of said tubes, the radius of curvature of the middle portion increasing with increasing length of said tubes, and the radius of curvature of substantially all of the surface intermediate the end and middle portions changing continuously and gradually therebetween, a cooling water channel within each of said drift tubes, means for supporting each of said drift tubes, said means including a vertical stem and a horizontal stem, said vertical stem having a water passageway therewithin communicating with said water channel, said horizontal stem having a water passageway communicating with said water channel, the horizontal stem having an electrical wire conduit thcrewithin, means for making good radiofrequency electrical contact between said vertical stern and said wall, including a bellows assembly, means for making good radiofrequency electrical contact between said horizontal stem and said wall, including a bellows assembly.

8. An apparatus for accelerating high velocity ions comprising a tank, a cylindrical resonant cavity of constant diameter within said tank, an ion source coaxially communicating with one end of said cavity and producing a longitudinal bunched beam of ions, a radiofrequency generator for energizing said cavity to produce a longitudinal field therein, a series of drift tubes with curved outer surfaces spaced apart to form gaps disposed within said cavity along the longitudinal axis thereof, said tubes increasing in length along the direction of movement of ions through saidcavity, substantially all of the outer surface of each of said tubes having an ellipsoidal curvature, the radius of curvature of the end portions of said surface decreasing with increasing length of said tubes, the radius of curvature of the middle portion increasing with increasing length of said tubes, and the radius of curvature of substantially all of the surface intermediate the end and middle portions changing continuously and gradually therebetween, means for supporting each of said drift tubes, said means including a vertical stem and a horizontal stem, said vertical stem being mounted on said tank by a vertical stem support assembly, said vertical stem support assembly including means for adjusting the vertical position of said drift tube and means permitting lateral and axial adjustment of the position of said drift tube, said horizontal stem being mounted on said tank by a horizontal stem support assembly, said horizontal stem support assembly including means for adjusting the lateral position of said drift tube, means for adjusting the axial position of said drift tube and means permitting the vertical adjustment of the position of said drift tube.

9. An apparatus for accelerating high velocity ions comprising a cylindrical resonant cavity of constant diameter, a radiofrequency generator for energizing said cavity to produce a longitudinal field therein, ion source means communicating with said cavity forinjecting a bunched beam of ions therein for longitudinal acceleration in said field, a; series of drift tubes with curved outer surfaces spaced apart to formgaps disposed within said cavity 12 along the longitudinal axis thereof, said tubes increasing. in length along the direction of movement of ions through said cavity, substantially all of the outer surface of each of said tubes having an ellipsoidal curvature, the radius of curvature of the end-portions of 'saidsurface increasing with increasing length of said tubes, the radius of curvature of the middle portion increasing with increasing length of said tubes and the radius of curvature of sub stantially all of the surface intermediate the end and middle portions changing continuously and gradually therebetween, and means for tuning said resonant cavity to its designed resonant frequency, said means including a plurality of spherical ball tuners within said cavity, each of said ball tuners being located outside a particular one of said gaps having its center in a radial plane at the mid point of said gap.

10. An apparatus for accelerating high velocity ions comprising a tank, a cylindrical resonant cavity of constant diameter within said tank, a radiofrequency generator for energizing said cavity to produce a longitudinal field therein, means injecting a bunched beam of ions into said cavity for longitudinal acceleration by said field, a series of drift tubes with curved outer surfaces spaced apart to form gaps disposed within said cavity along the longitudinal axis thereof, said tubes increasing in length along the direction of movement of ions through said cavity, substantially all of the outer surface of each of said tubes having an ellipsoidal curvature, the radius of curvature of the end-portions of said surface increasing with increasing length of said tubes, the radius of curvature of the middle portion increasing with increasing length of said tubes and the radius of curvature of substantially all of the surface intermediate the end and middle portions changing continuously and gradually therebetween, and means for controlling the temperature of said tank, said means including an entrance water manifold and an exit water manifold, a plurality of axial hairpin tubes bonded to said tank, said hairpin tubes terminating in said entrance and said exit water manifolds.

. 11. A linear ion accelerator comprising a cylindrical resonant cavity, a series of drift tubes with curved outer surfaces spaced apart to form gaps disposed within said cavity along the longitudinal axis thereof, a radiofrequency generator for energizing said cavity to produce a longitudinal field therein, means for injecting ions into said cavity at the increasing voltage portion of the excitation cycle of said field, said tubes increasing in length along the direction of movement of ions through said cavity, substantially all of the outer surface of each of said tubes having an ellipsoidal curvature, the radius of curvature of the end-portions of said surface increasing with increasing length of said tubes, the radius of curvature of the middleportion increasing with increasing length of said tubes and the radius of curvature of substantially all of the surface intermediate'the end and middle portions changing continuously and gradually therebetween.

12. An apparatus for accelerating high velocity ions comprising a cylindrical resonant cavity of constant diameter, a radiofrequency generator for energizing said cavity to produce a longitudinal electric field therein, means for injecting ions into said cavity at the increasing voltage portion of the excitation cycle of said field, a series of drift tubes spaced apart to form gaps disposed within said resonant cavity along the longitudinal axis thereof, said drift tubes having graded ellipsoidal outer surfaces thereby establishing said electric field in a manner'to optimize the shunt impedance of said cavity, each of said surfaces conforming to a surface of revolution whose cylindrical coordinates at each position thereof are related by the expression:

' 13 where r and z are the cylindrical coordinates of said position and B and E are the radial and axial components of said electric field on said drift tube at said coordinate position.

13. An apparatus for accelerating high velocity ions comprising a cylindrical resonant cavity of constant diameter and having axially aligned inlet and outlet openings at the opposite ends thereof for longitudinal passage of a bunched ion beam therethrough, a radiofrequency generator for energizing said cavity to produce a longitudinal electric field therein, a. series of drift tubes spaced apart to form gaps disposed within said resonant cavity along the longitudinal axis thereof, said drift tubes having graded ellipsoidal outer surfaces thereby establishing said electric field in a manner. to optimize the shunt impedance of said cavity, each of said surfaces conforming to a surface of revolution whose cylindrical coordinates at each position thereof are related by the expression:

where r and z are the cylindrical coordinates of said position and E, and B are the radial and axial components of said electric field on said drift tube at said coordinate position, said axial and radial components of the electric field being determined by two sets of equations each equation being a sum of finite number of cylindrical harmonies, the coefiicients of said cylindrical harmonics being determined by the boundary conditions, namely the tank radius, the gap between the drift tubes, the distance from gap center to gap center and the frequency of the driving radio-frequency generator.

References Cited in the file of this patent UNITED STATES PATENTS 

